1. Field of the Invention
The present invention relates to a control method of a rotational speed of a screw for an injection molding machine.
2. Description of the Related Art
Conventionally, in an injection molding machine, a screw is disposed within a heating cylinder of an injection apparatus such that it can rotate and can advance and retreat. By operating a drive mechanism, the screw can be rotated, and can be advanced and retreated. In a metering process, the screw is rotated, whereby a resin which is supplied from a hopper into the heating cylinder is melted through application of heat and transferred forward, and the molten resin is stored in a space located ahead of a screw head attached to the front end of the screw. In an injection process, the screw is caused to advance, whereby the molten resin, which is stored in the space located ahead of the screw head, is injected from an injection nozzle into the cavity of a mold apparatus so as to fill the cavity.
FIG. 1 is a conceptual view of a conventional injection apparatus.
In FIG. 1, reference numeral 11 denotes a heating cylinder. A screw 12 is disposed within the heating cylinder 11 such that it can rotate and can advance and retreat (move leftward and rightward in FIG. 1). An unillustrated injection nozzle is attached to the front end (left-hand end in FIG. 1) of the heating cylinder 11, and a nozzle hole is formed in the injection nozzle.
The rear end (right-hand end in FIG. 1) of the heating cylinder 11 is attached to a front injection support 61, and a rear injection support 62 is disposed a predetermined distance away from the front injection support 61. The front injection support 61 includes a box-like body 61a and a cover 61b. Rods 63 extend between the front injection support 61 and the rear injection support 62, for maintaining a predetermined distance therebetween. The front injection support 61, the rear injection support 62, and the rods 63 constitute an injection frame.
A circular connection member 64 is integrally attached to the rear end of the screw 12 via a coupler 59. A cylindrical support member 65 is attached to the connection member 64 by use of bolts bt1. The connection member 64 and the support member 65 constitute a rotary slide member 68, which unitarily rotates with the screw 12. A male spline 92 is formed on the outer circumferential surface of a rear end of the support member 65.
In order to transmit rotation to the rotary slide member 68, a cylindrical rotary member 78 is disposed while surrounding the rotary slide member 68, and a female spline 93 is formed on the inner circumferential surface of the rotary member 78. The female spline 93 has an axial length equivalent to the stroke of the screw 12. The rotary member 78 is supported by bearings b1 and b2 in such a manner as to be rotatable relative to the front injection support 61.
An electrically operated metering motor 70 is disposed. In a metering process, the metering motor 70 is operated to rotate the rotary slide member 68, whereas, in the injection process, the metering motor 70 generates reverse torque to stop the rotation of the rotary slide member 68. The metering motor 70 includes an unillustrated stator, an unillustrated rotor disposed radially inward of the stator, an output shaft 74, and an encoder 70a attached to the output shaft 74 and adapted to detect the rotational speed of the metering motor 70, and is controlled on the basis of a detection signal output from the encoder 70a. Each of the stator and the rotor includes a core, and a coil wound onto the core.
An output gear 75, a counter drive gear 76, a counter driven gear 77, and the rotary member 78 are disposed between the metering motor 70 and the rotary slide member 68. The output gear 75 is attached to the output shaft 74. The output gear 75 and the counter drive gear 76 are engaged with each other. The counter drive gear 76 and the counter driven gear 77 are engaged with each other. The counter driven gear 77 is attached to the rotary member 78 by use of bolts bt3.
The output gear 75, the counter drive gear 76, the counter driven gear 77, and the rotary member 78 transmit to the rotary slide member 68 rotation generated through operation of the metering motor 70. For such operation, the rotary slide member 68 is disposed in such a manner as to be nonrotatable and axially movable relative to the rotary member 78; and the outer circumferential surface of the connection member 64 and the inner circumferential surface of the rotary member 78 are in slidable contact with each other. That is, the female spline 93 formed on the inner circumferential surface of the rotary member 78 is spline-engaged with the above-described male spline 92 to be slidable relative thereto.
Accordingly, when the output shaft 74 is rotated through operation of the metering motor 70, rotation is transmitted to the rotary slide member 68 via the output gear 75, the counter drive gear 76, the counter driven gear 77, and the rotary member 78, whereby the rotary slide member 68 is rotated in the regular direction or in reverse as needed, and thus the screw 12 is rotated accordingly. When the metering motor 70 is stopped and is caused to restrain the output shaft 74 by force of restraint, the rotary slide member 68 is caused to stop rotating, so that the screw 12 is caused to stop rotating.
A ball screw 83, which includes a ball screw shaft 81 and a ball nut 82 and serves as a direction-of-motion changing section, is disposed rearward (rightward in FIG. 1) of the front injection support 61. The ball screw shaft 81 includes a small-diameter shaft portion 84, a large-diameter threaded portion 85, a connection portion to be connected to an injection motor 90, etc. which are sequentially formed from the front end of the ball screw shaft toward its rear end. An annular flange member 89 is externally fitted to the shoulder between the shaft portion 84 and the threaded portion 85.
The electrically operated injection motor 90 is fixed to the rear injection support 62 via a load cell 96. The injection motor 90 is operated in the injection process. Rotation generated by the injection motor 90 is transmitted to the threaded portion 85. The above-described ball screw 83 converts a rotary motion generated by the injection motor 90 to a rectilinear motion accompanied by rotation; i.e., to a rotary, rectilinear motion, and transmits the rotary, rectilinear motion to the rotary slide member 68.
Thus, the ball screw shaft 81 is supported, at its front end by bearings b7 and b8, in such a manner as to be rotatable and axially immovable relative to the rotary slide member 68, and is rotatably engaged with and supported by the ball nut 82 at its center. That is, the rotary slide member 68 is disposed in such a manner as to be rotatable relative to the ball screw 83 and axially movable relative to the rotary member 78. An unillustrated male screw is formed on a front end part of the shaft portion 84, and a bearing nut 80 is disposed while being engaged with the male screw. The bearing nut 80, together with a protrusion 65a formed on the inner circumferential surface of the support member 65, positions the bearing b7.
The ball nut 82 is fixedly attached to the rear injection support 62 via the load cell 96. The load cell 96 detects an injection force and a dwell pressure.
Accordingly, when rotation generated through operation of the injection motor 90 in the regular or reverse direction is transmitted to the ball screw shaft 81 via the connection portion, the ball screw shaft 81 is caused to advance or retreat while rotating, since the threaded portion 85 and the ball nut 82 are engaged with each other.
In the injection process or a like process, in which the rotary slide member 68 is caused to advance or retreat without rotation, stoppage of the operation of the metering motor 70 causes stoppage of the rotation of the rotary slide member 68, and the subsequent operation of the injection motor 90 causes the rotary slide member 68 to axially move. As a result, a rectilinear motion is transmitted to the screw 12, which is integrally attached to the rotary slide member 68, whereby the screw 12 is caused to advance (move leftward in FIG. 1).
Next, the operation of the thus-configured injection apparatus will be described.
First, in the metering process, when the metering motor 70 is operated, the rotation of the output shaft 74 is transmitted to the screw 12 via a transmission mechanism, which is composed of the output gear 75, the counter drive gear 76, the counter driven gear 77, the rotary member 78, etc., and the rotary slide member 68, to thereby rotate the screw 12 in the regular direction.
This rotation of the screw 12 causes unillustrated resin, which drops from an unillustrated hopper disposed on the heating cylinder 11, to advance along an unillustrated groove formed on the outer circumferential surface of the screw 12, and causes the screw 12 to retreat (move rightward in FIG. 1), whereby the resin is stored in a space located ahead of an unillustrated screw head attached to the front end of the screw 12. At this time, the force of retreat induced on the screw 12 causes the rotary slide member 68 to move relative to the rotary member 78; specifically, to retreat. As the rotary slide member 68 retreats, the ball screw shaft 81 is caused to retreat while rotating.
Next, in the injection process, the injection motor 90 is operated. The resultant rotation of the output shaft 94 is transmitted to the ball screw shaft 81 via the connection portion. The ball screw 83 converts the rotary motion to a rotary, rectilinear motion. As a result, the ball screw shaft 81 is caused to advance while rotating.
Then, when the metering motor 70 causes the rotary slide member 68 to stop rotating, the screw 12 is caused to advance without rotation, since the screw 12 is integrally attached to the rotary slide member 68.
Since the conventional injection apparatus requires the transmission mechanism and the rotary member 78 in order to transmit rotation generated by the metering motor 70 to the rotary slide member 68, noise arises from engagement of gears and other members, and the size thereof increases. Further, a large number of components are employed, resulting in emergence of torque loss and an increase in the cost of the injection apparatus.
Moreover, at the time of assembly, maintenance, etc. of the injection apparatus, the ball screw 83 is built into the injection apparatus along with the support member 65 and the bearings b7 and b8, and such building-in work must be performed in consideration of meshing engagement between the male spline 92 formed on the outer circumferential surface of the support member 65 and the female spline 93 formed on the inner circumferential surface of the rotary member 78. Therefore, when meshing engagement cannot be established properly at the spline portion, the building-in work requires a longer time, which deteriorates easiness of assembly, maintenance, etc.